1. Technical Field
This invention relates to a waste management facility for the treatment and disposal of spent potlining, spent from use in the electrolytic smelting of alumina in molten cryolite, by burning and glassification to make the spent potlining residue suitable for reuse or as a non-toxic waste for landfilling.
2. Background
Aluminum has the potential for being the most abundantly used metal in the world. Aluminum oxide itself makes up over eight percent of the solid portion of the Earth's crust. Commercial consumption of aluminum has increased rapidly in modern times, but has not reached the potential available for this multi-dimensional material.
Aluminum metal is produced today by the electrolytic reduction of alumina in a bath of molten cryolite. Commercial scale electrolytic aluminum smelting cells are huge steel tub-like containers. Inside each electrolytic cell is a cathode compartment, or cavity, lined with carbonaceous pre-baked cathode compartment blocks. Thermal insulation is placed between the compartment or cavity lining and the steel shell of the cell. Large steel bars serve as cathode current collectors embedded in the bottom portion of the cavity lining and extended out through openings in the shell to connect with the cathode bus.
The cathode compartment, or cavity, can be relatively large, e.g., such as of dimensions from 12 to 20 inches or more in depth and up to 10 feet wide and 30 feet long, depending on the type of cell and the amperage load for which it is designed. The carbonaceous cavity lining is provided with a depth dimension which can vary, e.g., such as from 6 to 10 inches or more on the sides and from 14 to 18 inches or more on the bottom. It is estimated that no fewer than 13,000 cells or pots of these dimensions are operational in the United States today for producing aluminum commercially.
The carbonaceous cell linings normally last for a long time, but when failure occurs, as from penetration of metal through the cathode compartment blocks which are dissolved, or from penetration of the metal out to the steel shell where it leaks out around the cathode current collectors, the entire carbonaceous cell or potlining is replaced.
In this manner, considerable amounts of spent potlining from the commercial electrolytic smelting of aluminum are accumulated each year and must be disposed. Some estimates have placed the amount of spent potlining from the industrial production of aluminum metal to be 120,000 tons annually.
The carbonaceous material cell linings, which form the bath-holding cavities of the alumina reduction smelting cells for producing aluminum in industrial quantities, become intercalated gradually by the penetration of the smelting cell bath materials, i.e., by penetration of cryolite and alumina. At the high temperatures of the electrolytic reduction smelting process, as the carbonaceous material of the lining gradually ages and degrades over time, it becomes saturated with these bath materials. The carbonaceous lining along with the insulating lining must then be removed. The material removed is called "spent potlining."
The spent potlining cannot be placed in a landfill immediately because of the cyanide content at levels ranging from about 1000 to 2000 parts per million. The spent potlining also has a high content of leachable fluorides at levels, e.g., such as of about 3000-8000 mg/l. Total fluorides present in the spent potlining are at levels ranging typically from about 14% to 18% by weight.
Conventional processes for reducing the fluoride content in spent potlining involve pyrohydrolysis of the spent cell material or potlining, e.g., such as in a fluidized bed reactor. Pyrohydrolysis is a process of contacting a fluidized bed of spent potlining with water or steam to react with the fluoride to form hydrogen fluoride. Sodium fluoride, cryolite, and other sodium compounds in the potlining react with the steam to form hydrogen fluoride vapors and sodium oxide vapors. The hydrogen fluoride vapors and sodium oxide vapors produced in the pyro-hydrolysis reactions can be recovered in liquid scrubbers.
Prior methods have used limestone in the form of calcium carbonate to react with the fluorides in the spent potlining at temperatures of about 700.degree. C. to 780.degree. C. to form a calcium fluoride. However, the final product has been found to have moderate levels of leachable fluorides.
Other developmental methods have attempted to remove the fluorides and fluorine balance of aluminum smelting plants by a process wherein the spent potlining is crushed, incinerated, and then pulverized so that fluorides can be extracted into water. After a filtration step, the pulverized material is introduced into a mixing tank, and sulfuric acid and calcium carbonate are added to produce a calcium fluoride. The resulting slurry is desiccated and the sludge is then sent to a disposal site.
The chemical composition of the electrolyte of the alumina reduction smelting bath, which is an important element in the low voltage aluminum production process, contains cryolite and fluorspar (calcium fluoride, CaF.sub.2). Accordingly, the large quantities of spent potlining from the industrial aluminum reduction cell linings thereby contain a significant amount of chemical values in the form of carbonaceous material, sodium fluoride, alumina (Al.sub.2 O.sub.3), a mixture of fluoride salts, and low levels of cyanide.
Several developmental methods have been tried for recovering the chemical values of the fluoride salts and alumina content from the spent potlining material.
U.S. Pat. No. 4,889,695 issued to Bush discloses a process for recovering aluminum fluoride, caustic, and carbon from spent potlining retrieved from an aluminum electrolytic reduction cell. Spent potlining is treated by leaching with a caustic solution to produce a fluoride-rich basic liquor and a carbonaceous solid residue. This is followed by contacting the fluoride-rich basic liquid with an acid bath of aluminum sulfate and sulfuric acid to increase aluminum fluoride product purity from about 53% by weight to about 92%.
U.S. Pat. No. 4,113,832 discloses a process wherein spent potlining is leached with a caustic solution followed by a precipitation of sodium fluoride by saturation of the leach liquor with a compound to suppress the solubility of the sodium fluoride in the leach liquor. The preferred compound is ammonia. Precipitated sodium fluoride is removed, and the leach solution saturated with ammonia is processed to remove the ammonia.
U.S. Pat. No. 4,444,740 discloses a process for removing and recovering fluoride values from spent potlining by leaching with a dilute caustic. The leachate is treated with a calcium compound to precipitate calcium fluoride.